The purpose of this study is to examine the influence of estrogen on serotonin (5HT) system and mood. We will study the effects of acute tryptophan (TRP) depletion (TRP is the 5-HT function) in healthy postmenopausal women before and after estrogen replacement treatment (ERT). We hypothesize that the postmenopausal women before ERT (low level of estrogen) will manifest increased vulnerability (lowering of mood) to acute TRP depletion.